The present invention relates to coating compositions for use in cathodic electrodeposition coating processes and methods of cathodic electrodeposition. More particularly, the invention provides resins for electrocoat coating compositions which contain a carbamate functional polymer (A) having one or more quaternary ammonium groups and a carbamate functional reactive additive (B) which is generated in situ during the preparation of said carbamate functional resin.
Coating compositions are widely in use today which utilize a variety of cure mechanisms. Among these are anodic and cathodic electrodeposition coating compositions and methods wherein a film-forming composition is deposited on a substrate under the influence of an applied electric potential. xe2x80x9cElectrodepositionxe2x80x9d as used herein refers to electrophoretic deposition. xe2x80x9cElectrocoatxe2x80x9d as used herein refers to both coating compositions used in electrophoretic deposition processes and to coating films obtained from electrophoretic deposition processes.
During electrodeposition, an ionically-charged polymer having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin, and applying an electrical potential between the substrate and a pole of opposite charge, usually a stainless steel electrode. This produces a relatively soft coating of low molecular weight on the substrate. This coating is usually converted to a hard high molecular weight coating by curing or crosslinking of the resin, usually upon exposure to elevated temperatures. In cathodic electrocoat, the workpiece being coated serves as the cathode.
One curing mechanism for prior art electrocoat compositions utilizes a melamine formaldehyde polymer-curing agent in the electrodepositable coating composition to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides good cure at relatively low temperatures (e.g., 132xc2x0 C.), but the crosslinked bonds contain undesirable ether linkages and the resulting coatings provide poor overall corrosion resistance.
For example, U.S. Pat. No. 4,501,833 discloses aminoplast curable cationic coating compositions that contain an onium salt-group containing polymer and a specific aminoplast curing agent. However, the performance of the cured coating is believed to be less than that desired by current commercial conditions.
In order to address some of the problems with melamine-crosslinked electrocoats, many users employ polyisocyanate crosslinkers to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides desirable urethane crosslink bonds, but it also entails several disadvantages. In order to prevent premature gelation of the electrodepositable coating composition, the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime, lactam, or alcohol).
Blocked polyisocyanates, however, require high temperatures (e.g., 176xc2x0 C. or more) to unblock and begin the curing reaction. The resulting electrocoats can also be susceptible to yellowing. Moreover, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, as well as increasing VOC. In addition, use of some the volatile blocking agents may give rise to environmental concerns. Finally, the volatile blocking agents account for significant and disadvantageous weight loss upon crosslinking.
In addition to the foregoing problems, prior art electrocoat compositions have sometimes lacked good flow at a particular dip viscosity and solids. It is thus desirable to obtain an electrocoat coating composition which would is free of the foregoing problems but also exhibits good flow at a particular dip viscosity and desirably high solids. It would also be advantageous to obtain such improvements with the add of a component which resulted in advantageous urethane linkages upon crosslinking, rather than undesirable ether linkages.
Finally, it is desirable to obtain electrocoat resin compositions, especially cationic resin compositions, which will satisfy the above concerns but which are also cost effective and commercially manufacturable.
There is thus a need in the art for electrodepositable coating compositions that can provide desirable urethane crosslink linkages, but avoid the problems of the prior art, especially those resulting from the use of blocked polyisocyanate curing agents. In particular, it is desirable to provide a cathodic electrodeposition coating composition capable of providing urethane linkages at low bake temperatures of 121xc2x0 C. or less with decreased weight loss upon crosslinking, while being free of isocyanates and the volatile blocking agents used with isocyanates.
The foregoing objects have unexpectedly been obtained with the use of an electrocoat resin composition comprising a carbamate functional polymer (A) having one or more quaternary ammonium groups and a carbamate functional reactive additive (B) obtained during and as a result of the production of said carbamate functional resin (A). The invention provides advantages in performance, cost and efficiency.
The invention also provides a method of making a electrocoat resin composition comprising (A) a polymer having at least one primary carbamate group and one or more quaternary ammonium groups and (B) a carbamate functional reactive additive, the method comprising reacting a monomeric polyisocyanate, and a compound comprising at least one group that is reactive with isocyanate and at least one carbamate group, so as to produce both (1) an intermediate product having at least one carbamate functional group and at least one isocyanate functional group, as well as (2) a carbamate functional reactive additive having no isocyanate functionality, reacting said intermediate product with a compound having at least one epoxy group and at least one isocyanate reactive group, said reaction occurring in the presence of the reactive additive so as to produce a carbamate functional resin having at least one epoxy group, reacting said at least one epoxy group with a tertiary amine compound in the presence of an acid to provide a carbamate functional resin having one or more quaternary ammonium groups, said reaction occurring in the presence of the reactive additive to provide a resin composition comprising (A) a carbamate functional resin having one or more quaternary ammonium groups and (B) a carbamate functional reactive additive.
Finally, the invention provides electrocoat coating compositions comprising the resin composition of the invention and a method of using said electrocoat coating compositions.